Pipe couplings of this kind for smooth plastics are already known under different trade names such as ISIFLO (reg. trademark) or PRK (reg. trademark), while couplings for smooth metal pipes are known under the trademarks KUTERLITE, VATETTF and CONEX. These known structures are burdened with different disadvantages, however.
The ISIFLO coupling includes clamping and sealing means consisting of several separate parts that must be inserted in a given order in a coupling housing, after which the coupling member, in this case consisting of a nut, is screwed into the coupling housing, the coupling then being fixed and sealed against the pipe. Since the different coupling details must be placed the right way round and in the right order in the coupling, there is always the risk that someone places a detail the wrong way round or in the wrong order, which seriously deteriorates the function of the coupling and also its ability to maintain good sealing for a long time. The latter property is naturally of great importance, since couplings of this kind are often buried to a great depth in the ground, resulting in considerable costs if the coupling must be exchanged or repaired.
The PRK coupling has other disadvantages connected with the fact that the coupling member a nut in this case as well, is made from plastics and formed such that, when it is screwed into the coupling housing, it is pressed around the pipe to give the intended sealing function. Thus, a part of the nut constitutes the clamping and sealing means in this case, which has the drawback that the nut will be very difficult to loosen after it has been tightened because the plastics details have been deformed in compression round the pipe. With this structure also, there is a risk of a deteriorated long-time sealing function of the coupling, should the nut have been incorrectly screwed into the housing, the metal threads in the housing thus being caused to cut up the considerably softer plastics threads of the nut, resulting in that the coupling is torn apart, e.g. when it is subjected to a high internal liquid pressure.
Each of the mentioned couplings has limited adaptability, and can only be utilized for connecting two pipe dimensions from one and the same side of the coupling. The pipe ends may not be of the same type, however-i.e., one must be smooth and the other threaded.